![HENA Earth's Ring Current July 2000 Geomagnetic Storm](https://upload.wikimedia.org/wikipedia/commons/8/87/HENA_Earth%27s_Ring_Current_July_2000_Geomagnetic_Storm.gif)
Energetic Neutral Atom (ENA) imaging, often described as "seeing with
atoms
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas, an ...
", is a technology used to create global images of otherwise invisible phenomena in the
magnetospheres
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynamo ...
of
planets
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a young ...
and throughout the
heliosphere
The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...
.
The
solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...
is composed of
plasma
Plasma or plasm may refer to:
Science
* Plasma (physics), one of the four fundamental states of matter
* Plasma (mineral), a green translucent silica mineral
* Quark–gluon plasma, a state of matter in quantum chromodynamics
Biology
* Blood pla ...
that is emitted from the Sun. The solar wind plasma is mostly composed of
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
, bare
electrons
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
and
protons
A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
, along with some other kinds of
nuclei, mostly
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
. The
space between solar systems is similar, but they come from other stars in our
galaxy
A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...
. These
charged particles
In physics, a charged particle is a particle with an electric charge. It may be an ion, such as a molecule or atom with a surplus or deficit of electrons relative to protons. It can also be an electron or a proton, or another elementary particle, ...
can be redirected by
magnetic fields
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
; for instance,
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
deflects these solar wind particles around the Earth. Occasionally, charged particles within plasmas steal electrons from neutral atoms they run into, making them neutral and not subject to large-scale
electromagnetic fields and waves. Still moving very fast, they tend to travel mostly in a straight line, subject to gravity. These are called Energetic Neutral Atoms. ENA images are constructed from the detection of these energetic neutral atoms.
Earth's
magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
preserves Earth's
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
and protects life on Earth from cell-damaging
radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
. This region of "
space weather
Space weather is a branch of space physics and aeronomy, or heliophysics, concerned with the time varying conditions within the Solar System, including the solar wind, emphasizing the space surrounding the Earth, including conditions in the ma ...
" is the site of
geomagnetic storms
A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field that interacts with the Earth's magnetic field.
The disturbance that d ...
that disrupt
communications systems
A communications system or communication system is a collection of individual telecommunications networks, transmission systems, relay stations, tributary stations, and terminal equipment usually capable of interconnection and interoperat ...
and pose
radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
hazards to humans traveling in
airplanes
An airplane or aeroplane (informally plane) is a fixed-wing aircraft that is propelled forward by thrust from a jet engine, propeller, or rocket engine. Airplanes come in a variety of sizes, shapes, and wing configurations. The broad spectr ...
(if both altitude and latitude are high) or in orbiting
spacecraft
A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
. A deeper understanding of this region is important.
Geomagnetic
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic fi ...
weather systems have been late to benefit from the
satellite
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope ...
imagery taken for granted in
weather forecasting
Weather forecasting is the application of science and technology forecasting, to predict the conditions of the Earth's atmosphere, atmosphere for a given location and time. People have attempted to predict the weather informally for millennia a ...
and
space physics Space physics, also known as solar-terrestrial physics or space-plasma physics, is the study of plasmas as they occur naturally in the Earth's upper atmosphere (aeronomy) and within the Solar System. As such, it encompasses a far-ranging number of ...
because their origins in
magnetospheric
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynamo. ...
plasma
Plasma or plasm may refer to:
Science
* Plasma (physics), one of the four fundamental states of matter
* Plasma (mineral), a green translucent silica mineral
* Quark–gluon plasma, a state of matter in quantum chromodynamics
Biology
* Blood pla ...
frequency present the added problem of invisibility.
[
The ]heliosphere
The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...
protects the entire Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
from the majority of cosmic rays
Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
but is so remote that only an imaging technique such as ENA imaging will reveal its properties. The heliosphere's structure is due to the invisible interaction between the solar wind and cold gas from the local interstellar medium
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
.[
The creation of ENAs by ]space plasma
The interplanetary medium (IPM) or interplanetary space consists of the mass and energy which fills the Solar System, and through which all the larger Solar System bodies, such as planets, dwarf planets, asteroids, and comets, move. The IPM sto ...
was predicted, but their discovery was both deliberate and serendipitous. While some early efforts were made at detection, their signatures also explained inconsistent findings by ion detectors in regions of expected low-ion populations. Ion detectors were co-opted for further ENA detection experiments in other low-ion regions.[ However, the development of dedicated ENA detectors entailed overcoming significant obstacles in both skepticism and technology.][
Although ENAs were observed in space from the 1960s through 1980s,][ the first dedicated ENA camera was not flown until 1995 on the Swedish Astrid-1 satellite,][
] to study Earth's magnetosphere wind.
Today, dedicated ENA instruments have provided detailed magnetospheric
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynamo. ...
images from Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
, Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
, Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
, and Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
. Cassini's ENA images of Saturn revealed a unique magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
with complex interactions that have yet to be fully explained.[ The IMAGE mission's three dedicated ENA cameras observed Earth's ]magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
from 2000–2005[ while the ]TWINS Mission
''Twins Mission'' () is a 2007 Hong Kong martial arts-action-comedy film directed by action choreographer Kong Tao-Hoi and starring Sammo Hung, Gillian Chung, Charlene Choi and Wu Jing among others. The film is a bit of a spoof of the popu ...
, launched in 2008, provides stereo ENA imaging of Earth's magnetosphere using simultaneous imaging from two satellites.
The first ever images of the heliosphere boundary, published in October 2009, were made by the ENA instruments aboard the IBEX
An ibex (plural ibex, ibexes or ibices) is any of several species of wild goat (genus ''Capra''), distinguished by the male's large recurved horns, which are transversely ridged in front. Ibex are found in Eurasia, North Africa and East Africa. ...
and Cassini spacecraft. These images are important because they challenge existing theories about the region.
Creation of ENAs
The most abundant ion in space plasma is the hydrogen ion
A hydrogen ion is created when a hydrogen atom loses or gains an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particle ...
—a bare proton with no excitable electrons to emit visible photons. The occasional visibility of other plasma ions is not sufficient for imaging purposes.[ ENAs are created in charge-exchange collisions between hot solar plasma ions and a cold neutral background gas. These charge-exchange processes occur with high frequency in planetary ]magnetospheres
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynamo ...
and at the edge of the heliosphere
The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...
.
Charge exchange
In a charge-exchange collision between a high energy plasma ion and a cold neutral atom, the ion 'gives' electrons by the neutral atom, producing a cold ion and an energetic neutral atom (ENA). This chemical reaction can be described as:
I1+ + A2 → A1 + I2+
where I1+ is the plasma ion, A2 is a low energy background neutral atom, A1 is the energetic neutral atom (ENA) and I2+ is the lower energy ion.
Species 1 and 2 in this charge-exchange reaction may be the same, such as in proton–hydrogen charge-exchanges:
H+ + H → H + H+
Additionally, multiple electrons may be exchanged during the ion / neutral reaction. One example of this is the alpha-helium charge-exchange:
He2+ + He → He + He2+
Due to its charge
Charge or charged may refer to:
Arts, entertainment, and media Films
* '' Charge, Zero Emissions/Maximum Speed'', a 2011 documentary
Music
* ''Charge'' (David Ford album)
* ''Charge'' (Machel Montano album)
* ''Charge!!'', an album by The Aqu ...
neutrality, the ENA produced in this reaction is only subject to gravitation
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stron ...
al forces. This is in contrast to charged particles (ions, protons or electrons) within plasmas that are also subject to electromagnetic forces. Gravitational influences can generally be ignored in space plasmas, so it is common to assume that the ENA preserves the vector
Vector most often refers to:
*Euclidean vector, a quantity with a magnitude and a direction
*Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism
Vector may also refer to:
Mathematic ...
momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...
of the original pre-interaction plasma ion.[
Some ENAs are lost in further charge-exchange, electron collisions and photoionization and polarization, but a great many travel very long distances in space completely undisturbed.][
Although ]plasma recombination
Plasma recombination is a process by which positive ions of a plasma capture a free (energetic) electron and combine with electrons or negative ions to form new neutral atoms (gas). Recombination is an exothermic reaction, meaning heat releasing ...
and neutral atom acceleration by the solar gravitation may also contribute to an ENA population under certain conditions, the main exception to this creation scenario is the flux
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...
of interstellar gas
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
, where neutral particles from the local interstellar medium
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
penetrate the heliosphere with considerable velocity, which classifies them as ENAs as well.[
]
Solar eruptions
Solar flares
A solar flare is an intense localized eruption of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other solar phe ...
and coronal mass ejections
A coronal mass ejection (CME) is a significant release of plasma (physics), plasma and accompanying magnetic field from the Sun's Solar corona, corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar ac ...
(CMEs) are the result of eruptions on the surface of the Sun, which may provide another source of ENAs. The STEREO
Stereophonic sound, or more commonly stereo, is a method of sound reproduction that recreates a multi-directional, 3-dimensional audible perspective. This is usually achieved by using two independent audio channels through a configuration ...
spacecraft detected neutral hydrogen atoms with energies in the 2–5 MeV range from the flare/CME SOL2006-12-05. These particles were not detected with an instrument designed to see ENAs, but there was sufficient ancillary data to make the observation quite unambiguous.
Accelerating ENAs without ionizing them would be difficult, so the observed ENAs were interpreted to have resulted from charge exchange between solar energetic particles
Solar energetic particles (SEP), formerly known as solar cosmic rays, are high-energy, charged particles originating in the solar atmosphere and solar wind. They consist of protons, electrons and heavy ions with energies ranging from a few ten ...
(SEP) emitted from the flare/CME with helium atoms in the solar wind. Charge exchange then occurred between the extremely fast SEP protons and the slower solar wind helium atoms, to create the highly neutral hydrogen atoms and slower helium ions. The resulting ENAs propagated through space without being bound to follow the Parker Spiral
The heliospheric current sheet, or interplanetary current sheet, is a surface separating regions of the heliosphere where the interplanetary magnetic field points toward and away from the Sun. A small electrical current with a current density of ...
, so were observed near the Earth before the helium ions that were created in this reaction. This event, which occurred in 2006, was the first observation of ENAs produced by solar eruptions.
Species of ENAs
Proton–hydrogen charge-exchange collisions are often the most important process in space plasma because hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
is the most abundant constituent of both plasmas and background gases. Hydrogen charge-exchange occurs at very high velocities
Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity is a ...
involving little exchange of momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...
, so the resulting ENAs travel at high speeds.
In general, only a few species are important for ENA formation, namely hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
, helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
, oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
and sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
:
* Atomic hydrogen dominates Earth's neutral particle environment from altitudes of to . This altitude variation occurs as the solar cycle
The solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a nearly periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surfa ...
varies from solar minimum
Solar minimum is the regular period of least solar activity in the Sun's 11-year solar cycle. During solar minimum, sunspot and solar flare activity diminishes, and often does not occur for days at a time. On average, the solar cycle takes abou ...
to solar maximum
Solar maximum is the regular period of greatest solar activity during the Sun's 11-year solar cycle. During solar maximum, large numbers of sunspots appear, and the solar irradiance output grows by about 0.07%. On average, the solar cycle takes ...
.
* The interstellar and solar winds are mainly protons (H+), with the solar wind also containing ~5% alpha particles (He2+ ).
* Planetary magnetospheric plasma consists mostly of protons with some helium and oxygen.
* Helium and oxygen are also important species in the Earth's inner magnetosphere, particularly in regions of ionospheric
The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays a ...
outflow.
* Jupiter's magnetosphere
The magnetosphere of Jupiter is the cavity created in the solar wind by the planet's magnetic field. Extending up to seven million kilometers in the Sun's direction and almost to the orbit of Saturn in the opposite direction, Jupiter's magnet ...
additionally contains sulfur ions, due to volcanic activity its moon Io.
The corresponding neutral gases corresponding to these regions of space are:
* the geocorona
The geocorona is the luminous part of the outermost region of the Earth's atmosphere, the exosphere. It is seen primarily via far-ultraviolet light ( Lyman-alpha) from the Sun that is scattered from neutral hydrogen. It extends to at minimum 1 ...
for the Earth's magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
wind
* a planetary exosphere
The exosphere ( grc, ἔξω "outside, external, beyond", grc, σφαῖρα "sphere") is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the densit ...
for a planetary magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
* the local interstellar medium
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
in the boundary region of the heliosphere
The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...
at the termination shock
The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interst ...
and the heliopause.[
]
Energies
ENAs are found everywhere in space[ and are directly observable at energies from 10 eV to more than 1 M eV.][ Their energies are described more with reference to the instruments used for their detection than to their origins.
No single particle analyzer can cover the entire energy interval from 10 eV to beyond 1 M eV. ENA instruments are roughly divided into those that can detect low, medium and high energies in overlapping groups that can be arbitrary and vary from author to author. The low, medium and high energy ranges from one author is shown in the graph][ along with the energy ranges for the three instruments aboard the IMAGE satellite:
* a high energy instrument, HENA measuring 10–500 Kev energy to study Earth's ]ring current
A ring current is an electric current carried by charged particles trapped in a planet's magnetosphere. It is caused by the longitudinal drift of energetic (10–200 k eV) particles.
Earth's ring current
Earth's ring current is responsible f ...
;
* a medium ENA instrument, MENA measuring 1–30 Kev to study the plasma sheet
In the magnetosphere, the plasma sheet is a sheet-like region of denser (0.3-0.5 ions/cm3 versus 0.01-0.02 in the lobes) hot plasma and lower magnetic field near the equatorial plane, between the magnetosphere's north and south lobes.
A magneto ...
; and
* a low ENA instrument measuring between 10 eV and 500 eV to study the ionospheric
The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays a ...
source of ions flowing from the polar cap
Polar may refer to:
Geography
Polar may refer to:
* Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates
* Polar climate, the c ...
.[
Atoms are usually considered ENAs if they have kinetic energies of gases clearly higher than that can be reached by typical ]thermodynamic
Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of ther ...
planetary atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
s, which is usually in excess of 1 eV. This classification is somewhat arbitrary, being driven by the lower limits of ENA measurement instrumentation. The high end limitations are imposed by both measurement techniques and for scientific reasons.[
]
Magnetospheric ENA imaging
Magnetospheres are formed by the solar wind plasma flow around planets with an intrinsic magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
(Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
, Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
, Saturn
Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
, Uranus
Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...
, and Neptune
Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
), although planets and moons lacking magnetic fields may sometimes form magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
-like plasma structures.[ The ionospheres of weakly magnetized planets such as ]Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
and Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
set up currents that partially deflect the solar wind flow around the planet.[ ENAs have been observed in a range of planetary magnetospheres throughout our solar system.
Although ]magnetospheric
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynamo. ...
plasma
Plasma or plasm may refer to:
Science
* Plasma (physics), one of the four fundamental states of matter
* Plasma (mineral), a green translucent silica mineral
* Quark–gluon plasma, a state of matter in quantum chromodynamics
Biology
* Blood pla ...
fluctuation has very low densities; e.g. near Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
's moon Europa
Europa may refer to:
Places
* Europe
* Europa (Roman province), a province within the Diocese of Thrace
* Europa (Seville Metro), Seville, Spain; a station on the Seville Metro
* Europa City, Paris, France; a planned development
* Europa Cliff ...
, plasma pressures are about 10−13 bar, compared to 1 bar at Earth's surface, and are responsible for magnetospheric
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynamo. ...
dynamics and emissions. For example, geomagnetic storms
A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field that interacts with the Earth's magnetic field.
The disturbance that d ...
create serious disturbances in Earth's cable communications systems, navigational systems and power distribution systems.
The strength and orientation of the magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
with respect to solar wind flow determines the shape of the magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
. It is usually compressed on the day side and elongated at the night side.[
]
Earth's magnetosphere
The Earth's magnetic field
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic f ...
forms a magnetic cavity within the solar wind, called the magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
. Energetic particles in this region of space can have major space weather
Space weather is a branch of space physics and aeronomy, or heliophysics, concerned with the time varying conditions within the Solar System, including the solar wind, emphasizing the space surrounding the Earth, including conditions in the ma ...
impacts, including damaging satellites that orbit the Earth and presenting hazards for astronauts. ENA imaging is important to understand the dynamics of these energetic particles, which then allows for mitigation of these space weather effects.
![ISEE-1](https://upload.wikimedia.org/wikipedia/commons/2/27/ISEE-1.jpg)
ENA detection in Earth's magnetosphere
The first dedicated ENA instrument was launched on a Nike–Tomahawk sounding rocket from Fort Churchill, Manitoba
Manitoba ( ) is a Provinces and territories of Canada, province of Canada at the Centre of Canada, longitudinal centre of the country. It is Canada's Population of Canada by province and territory, fifth-most populous province, with a population o ...
, Canada
Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...
. This experiment was followed by the launch of a similar instrument on a Javelin
A javelin is a light spear designed primarily to be thrown, historically as a ranged weapon, but today predominantly for sport. The javelin is almost always thrown by hand, unlike the sling, bow, and crossbow, which launch projectiles with th ...
sounding rocket in 1970 to an altitude of at Wallops Island
Wallops Island is a island in Accomack County, Virginia, part of the Virginia Barrier Islands that stretch along the eastern seaboard of the United States of America. It is just south of Chincoteague Island, a popular tourist destination.
W ...
off the coast of Virginia
Virginia, officially the Commonwealth of Virginia, is a state in the Mid-Atlantic and Southeastern regions of the United States, between the Atlantic Coast and the Appalachian Mountains. The geography and climate of the Commonwealth ar ...
. In 1972 and 1973, the presence of ENA signatures explained inconsistencies in measurements by the IMP-7 and 8 satellites.[
ENA data from the NASA/ESA ISEE 1 satellite enabled the construction of the first global image of the storm time ]ring current
A ring current is an electric current carried by charged particles trapped in a planet's magnetosphere. It is caused by the longitudinal drift of energetic (10–200 k eV) particles.
Earth's ring current
Earth's ring current is responsible f ...
in 1982. This was a breakthrough that paved the way for the use of ENAs as a powerful imaging technique. ENAs were also detected during the 1982 magnetic storm
A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field that interacts with the Earth's magnetic field.
The disturbance that d ...
by SEEP instrument on the NASA S81-1 spacecraft. In 1989, the exospheric hydrogen atom
A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral atom contains a single positively charged proton and a single negatively charged electron bound to the nucleus by the Coulomb force. Atomic hydrogen consti ...
population around Earth was extensively studied by the NASA Dynamic Explorer (DE-1) satellite.[
]
An instrument with a dedicated high-energy ENA detection channel was flown on the 1991 NASA CRRES
The Combined Release and Radiation Effects Satellite (CRRES) was launched on July 25, 1990, into a geosynchronous transfer orbit (GTO) for a nominal three-year mission to investigate fields, plasmas, and energetic particles inside the Earth's ...
satellite. A more sophisticated high energy particle instrument was launched on the 1992 NASA/ISAS GEOTAIL
Geotail was a satellite that observed the Earth's magnetosphere. It was developed by Japan's ISAS in association with the United States' NASA, and was launched by a Delta II rocket on 24 July 1992 from Cape Canaveral Air Force Station.
The prim ...
spacecraft dedicated to observing Earth's magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
astronomy. Precipitating ENAs can be studied from a low Earth orbit and were measured "looking out" by CRRES
The Combined Release and Radiation Effects Satellite (CRRES) was launched on July 25, 1990, into a geosynchronous transfer orbit (GTO) for a nominal three-year mission to investigate fields, plasmas, and energetic particles inside the Earth's ...
and the 1995 Swedish ASTRID satellites.[
The new millennium saw ENA Imaging coming into its own. Extensive and detailed observations of the Earth's magnetosphere were made with three ENA instruments aboard the NASA ]IMAGE
An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensiona ...
Mission from 2000 – 2005.[ In July 2000, a set of ENA images of the Earth's ring current were made during a geomagnetic storm. (See image at the top of the page.) The storm was triggered by a fast coronal mass ejection that erupted from the ]Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
on July 14, 2000 and arrived at Earth the next day.
Launched in 2008, the NASA TWINS Mission (two wide-angle Imaging Neutral-atom Spectrometers
A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...
) provides the capability for stereoscopically imaging the magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
. By imaging ENAs over a broad energy range (~1–100 keV) using identical instruments on two widely spaced high-altitude, high-inclination spacecraft, TWINS enables 3-dimensional visualization and the resolution of large scale structures and dynamics within the magnetosphere.
Planetary and other magnetospheres
Magnetospheres of other planets have been studied by flyby spacecraft by orbiters, landers and by Earth-based observations.[
]
Earth's Moon
In February 2009, the ESA SARA LENA instrument aboard India's Chandrayaan-1
Chandrayaan-1 (, ) was the first Indian lunar probe under the Chandrayaan program. It was launched by the Indian Space Research Organisation in October 2008, and operated until August 2009. The mission included a lunar orbiter and an impact ...
detected hydrogen ENAs sputtered from the lunar surface by solar wind protons. Predictions had been that all impacting protons would be absorbed by the lunar regolith
Lunar soil is the fine fraction of the regolith found on the surface of the Moon. Its properties can differ significantly from those of terrestrial soil. The physical properties of lunar soil are primarily the result of mechanical disintegra ...
but, for an as yet unknown reason, 20% of them are bounced back as low energy hydrogen ENAs. It is hypothesized that the absorbed protons may produce water and hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
s in interactions with the regolith. The Moon has no magnetosphere.
Mercury
Launched in 2018, the ESA BepiColombo
BepiColombo is a joint mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) to the planet Mercury. The mission comprises two satellites launched together: the Mercury Planetary Orbiter (MPO) and ''Mio'' ...
mission includes ENA instruments to further its objective to study the origin, structure and dynamics of Mercury's magnetic field. The LENA instrument will resemble the SARA instrument sent to Earth's Moon. In addition to magnetospheric ENAs, sputtering from Mercury's surface is also expected.
Venus
Launched in 2005, the ESA VEX (Venus Express
''Venus Express'' (VEX) was the first Venus exploration mission of the European Space Agency (ESA). Launched in November 2005, it arrived at Venus in April 2006 and began continuously sending back science data from its polar orbit around Venus. ...
) mission's ASPERA (Energetic Neutral Atoms Analyser) consists of two dedicated ENA detectors.[ In 2006 ENA images were obtained of the interaction between the solar wind and the Venusian upper atmosphere, showing massive escape of planetary oxygen ions.
]
Mars
Launched in 2003, the ESA MEX (Mars Express
''Mars Express'' is a space exploration mission being conducted by the European Space Agency (ESA). The ''Mars Express'' mission is exploring the planet Mars, and is the first planetary mission attempted by the agency. "Express" originally ref ...
) mission's ASPERA instrument has obtained images of the solar wind interacting with the upper Martian atmosphere.[ The 2004 observations show solar wind plasma and accelerated ions very deep in the ionosphere, down to . above the dayside planetary surface—evidence for solar wind atmospheric erosion.
]
Jupiter
The GAS instrument on the ESA/NASA Ulysses
Ulysses is one form of the Roman name for Odysseus, a hero in ancient Greek literature.
Ulysses may also refer to:
People
* Ulysses (given name), including a list of people with this name
Places in the United States
* Ulysses, Kansas
* Ulysse ...
, launched in 1990, produced unique data on interstellar helium characteristics and ENAs emitted from Jupiter's Io torus.[ On its Jupiter flyby in 2000, the NASA/ESA/ASI Cassini's INCA instrument confirmed a neutral gas torus associated with ]Europa
Europa may refer to:
Places
* Europe
* Europa (Roman province), a province within the Diocese of Thrace
* Europa (Seville Metro), Seville, Spain; a station on the Seville Metro
* Europa City, Paris, France; a planned development
* Europa Cliff ...
. Cassini's ENA images also showed Jupiter's magnetosphere
The magnetosphere of Jupiter is the cavity created in the solar wind by the planet's magnetic field. Extending up to seven million kilometers in the Sun's direction and almost to the orbit of Saturn in the opposite direction, Jupiter's magnet ...
to be dominated by hydrogen atoms
A hydrogen atom is an atom of the chemical element hydrogen. The Electric charge, electrically neutral atom contains a single positively charged proton and a single negatively charged electron bound to the nucleus by the Coulomb force. Atomic ...
ranging from a few to 100 keV. The atoms are emitted from the planet's atmosphere and from neutral gas tori near the inner Galilean moons
The Galilean moons (), or Galilean satellites, are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in December 1609 or January 1610, and recognized by him as satellites of Jupiter ...
. A population of heavier ions was also detected, indicating a significant emission of oxygen and/or sulfur from Jupiter's magnetosphere.
Saturn
The first dedicated ENA camera was flown on the NASA/ESA/ASI Cassini mission Cassini may refer to:
People
* Cassini (surname)
* Oleg Cassini (1913-2006), American fashion designer
:Cassini family:
* Giovanni Domenico Cassini (1625–1712), Italian mathematician, astronomer, engineer, and astrologer
* Jacques Cassini (167 ...
, launched in 1997 to study Saturn's magnetosphere.[
Saturn's main radiation belt was measured beginning at an altitude from its surface and reaching out to . Cassini also detected a previously unknown inner belt nearer its surface that is about thick.
The dynamics of Saturn's magnetosphere are very different from Earth's. Plasma co-rotates with Saturn in its ]magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
. Saturn's strong magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
and rapid rotation create a strong co-rotational electric field
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
that accelerates plasma in its magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
until it reaches rotation speeds near that of the planet. Because Saturn's moons are essentially 'sitting still' in this very high speed flow, a complex interaction between this plasma and the atmosphere of the moon Titan was observed.[
]
Titan
Cassini's MIMI-INCA ENA instrument has observed Titan on many occasions revealing the structure of the magnetospheric interaction with Titan's dense atmosphere. Several studies have been performed on Titan's ENA emissions.
Uranus and Neptune
NASA's Voyager 2
''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, ''Voyager 1'', on a ...
took advantage of its orbit to explore Uranus
Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...
and Neptune
Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
, the only spacecraft to ever have done so. In 1986 spacecraft found a Uranian magnetic field that is both large and unusual. More detailed investigations have yet to be carried out.
Heliosphere ENA imaging
The heliosphere
The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...
is a cavity built up by the solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...
as it presses outward against the pressure of the local interstellar medium
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
(LISM). As the solar wind is a plasma, it is charged and so carries with it the Sun's magnetic field. So the heliosphere can be conceptualized as the Solar System's magnetosphere. The edge of the heliosphere is found far beyond the orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...
of Pluto
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
where diminishing solar wind pressure is stopped by the pressure from the LISM.[
]
The background neutral gas for ENA production at the heliosphere boundary comes predominantly from interstellar gas penetrating the heliosphere. A tiny amount comes from solar wind neutralization of interplanetary dust near the sun. The heliosphere boundaries are invisible and fluctuating. Although the densities are low, the enormous thickness of the heliosphere
The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...
makes it a dominant source of ENAs, aside from planetary magnetospheres.[ Because of the strong dependence of ENA characteristics on heliosphere properties, remote ENA imaging techniques will provide a global view of the structure and dynamics of the heliosphere unattainable by any other means.][
The first glimpse of this view was announced in October, 2009, when the NASA IBEX Mission, returned its first image of the unexpected ENA ribbon at the edge of the ]heliosphere
The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...
. Results revealed a "very narrow ribbon that is two to three times brighter than anything else in the sky" at the edge of the heliosphere that was not detected by Voyager 1
''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin ''Voyager 2'', ''Voya ...
or Voyager 2
''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, ''Voyager 1'', on a ...
in the region. [
Cassini also ENA-imaged the heliosphere and its results complement and extend the IBEX findings, making it possible for scientists to construct the first comprehensive sky map of the heliosphere. Preliminary Cassini data suggest the heliosphere may not have the comet-like shape predicted by existing models but that its shape may be more like a large, round bubble.][
Estimates for size of the heliosphere vary between 150 – 200 AU.][ It is believed that ]Voyager 1
''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin ''Voyager 2'', ''Voya ...
passed the heliosphere's termination shock
The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interst ...
in 2002 at approx. 85 – 87 AU[ while ]Voyager 2
''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, ''Voyager 1'', on a ...
passed the termination shock in 2007 at about 85 AU. Others place the termination shock at a mean distance of ≈100 AU.[ Because the ]solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...
varies by a factor of 2 during the 11 year solar nuclear cycle, there will be variations in the size and shape of the heliosphere, known as heliosphere "breathing."[
The huge distances involved mean we will never accumulate a large number of ''in situ'' measurements of the various layers of the heliosphere. Voyager 1 and 2 took 27 yrs. and 30 yrs. respectively to arrive at the termination shock. It is worth noting that for large distances to the object, high energy (velocity) and slower ENAs emitted simultaneously would be detected at different times. This time difference varies from 1 - 15 minutes for observing Earth's magnetosphere from a high altitude spacecraft to more than a year for imaging the heliospheric boundary from an Earth orbit.][
]
ENA instruments
Although the study of ENAs promised improvements in the understanding of global magnetospheric and heliosphere processes, its progress was hindered due to initially enormous experimental difficulties.
In the late 1960s, the first direct ENA measurement attempts revealed the difficulties involved. ENA fluxes are very weak, sometimes less than 1 particle per cm2 per second and are typically detected by secondary electron emission upon contact with a solid surface. They exist in regions containing ultraviolet (UV) and extreme ultraviolet (EUV) radiation at fluxes 100 times greater than produce similar emissions.[
An ENA instrument ideally would also specifically:
# prevent the entrance of charged particles
# suppress background light (photons), particularly UV and EUV radiation
# measure mass and energy of incoming ENAs
# determine trajectories of incoming ENAs
# measure ENA fluxes from 10−3 to 105 per cm2 per steradian per second
# measure ENAs ranging in energy from a few eV up to >100 Kev][
The challenge to remote sensing via ENAs lies in combining mass spectrometry with the imaging of weak particle fluxes within the stringent limitations imposed by an application on a spacecraft.][
]
Medium and high energy ENA cameras
It became clear very early that to succeed, instruments would have to specialize in specific ENA energies. The following describes, in very simplified terms, a typical instrument function for high (HENA) or medium (MENA) energy instrument, with differences noted. The accompanying illustration is of the HENA camera flown on the NASA IMAGE mission and the description that follows most closely resembles IMAGE mission instruments.
Collimator
A set of electrostatic plates deflect charged particles away from the instrument and collimates the beam of incoming neutral atoms to a few degrees.
Photon rejection & time of flight (TOF)
HENA: TOF is determined by a coincidence detection requirement that turns out to be efficient at eliminating photon background noise as well. An ENA passes through a thin film to a particle energy detector with its energy nearly completely preserved. At the same time, electrons forward scattered from the film are electrostatically
Electrostatics is a branch of physics that studies electric charges at rest ( static electricity).
Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for ambe ...
ionization deflected to a detector to create a start pulse. The ENA arriving at its solid state detector (SSD) creates the end pulse and its impact position yields its trajectory and therefore path length. The start and stop signals enable TOF to be determined.
If the electrons are scattered by incoming photons, no ENA will be detected to create the stop pulse. If no stop pulse is sensed within an established time appropriate to the energy of the expected particles, the start pulse is discarded.
MENA: Medium energy ENAs would lose too much energy penetrating the film used in the HENA instrument. The thinner film required would be vulnerable to damage by incident UV and EUV. Therefore, photons are prevented from entering the instrument by using a gold diffraction grating. An ultra thin carbon film is mounted on the back of the grating. ENAs pass through the grating and the film to impact a solid state detector (SSD), scattering electrons and allowing path length and TOF determinations as for the HENA above.
Knowing path length and TOF enables velocity to be determined.
Energy
The solid state detector (SSD) impacted by the ENA after it passes through the foil registers its energy. The small energy loss due to passing through the foil is handled by instrument calibration.
Mass
Knowing the energy and velocity, the mass of the particle can be calculated from energy = mv2/2. Alternatively, the number of scattered electrons detected can also serve to measure the mass of the ENA.[
Mass resolution requirements are normally modest, requiring at most distinguishing among hydrogen (1 AMU), helium (4 AMU), and oxygen (16 AMU) atoms with sulfur (32 amu) also expected in Jupiter's magnetosphere.][
]
2D and 3D imaging
Usually, obtaining images from a spinning spacecraft provides the second dimension of direction identification. By combining synchronized observations from two different satellites, stereo imaging becomes possible.[ Results from the TWINS Mission are eagerly awaited, as two viewing points will provide substantially more information about the 3-D nature of Earth's magnetosphere.
]
Low energy ENA cameras
While the collimator is similar, low-energy instruments such as the NASA GSFC LENA use a foil-stripping technique. Incident ENAs interact with a surface such as tungsten to generate ions that are then analyzed by an ion spectrometer.[
Because of the need to detect atoms sputtered from the lunar surface as well lighter ENAs, the ESA LENA on the ]Chandrayaan-1
Chandrayaan-1 (, ) was the first Indian lunar probe under the Chandrayaan program. It was launched by the Indian Space Research Organisation in October 2008, and operated until August 2009. The mission included a lunar orbiter and an impact ...
incorporated a mass spectrometer designed to resolve heavier masses including sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable iso ...
, potassium
Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...
, and iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
.[
]
Future
As of 2005, a total of only six dedicated ENA detectors had been flown.[ The launch of instruments aboard in the TWINS and IBEX missions brings the total to nine in 2009 – a 50% increase in only 4 years. Space plasma observation using ENA imaging is an emerging technology that is finally coming into its own.
Several improvements are still needed to perfect the technique. Although the angular resolution has now decreased to a few degrees and different species can be separated, one challenge is to expand the energy range upwards to about 500 Kev. This high energy range covers most of the plasma pressure of Earth's inner magnetosphere as well as some of the higher-energy radiation belts so is desirable for terrestrial ENA imaging.][
For lower energy ENAs, below 1 Kev, the imaging techniques are completely different and rely on the spectroscopic analysis of ions stripped from a surface by the impinging ENA. Improvements in sub-Kev measurements will be needed to image Mercury's magnetosphere due to the consequences of its smaller magnetic field and its smaller geometry.][
]
Importance for Earth
In addition to the obvious intellectual benefits brought by increased understanding of our space environment, there are many practical motivations for enhancing our knowledge of space plasma.
The heliosphere is a protective cocoon for the Solar System, just as the Earth's magnetosphere is a protective cocoon for the Earth. The insight provided by ENAs into the behavior of space plasma improves our understanding of these protective mechanisms.
Without the magnetosphere, Earth would be subject to direct bombardment by the solar wind and may be unable to retain an atmosphere. This, plus increased exposure to solar radiation means that life on Earth as we know it would not be possible without the magnetosphere. Similarly, the heliosphere protects the Solar System from the majority of otherwise damaging cosmic rays, with the remainder being deflected by the Earth's magnetosphere.
Although most orbiting satellites are protected by the magnetosphere, geomagnetic storms induce currents in conductors that disrupt communications both in space and in cables on the ground. Better understanding of the magnetosphere and the ring current and its interaction with the solar wind during high solar activity will allow us to better protect these assets.
Astronauts on deep space missions will not have Earth's protections so understanding the factors that may affect their exposure to cosmic rays and the solar wind is critical to manned space exploration.
Notes
Astronomers measure distances within the Solar System in astronomical unit
The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits t ...
s (AU). One AU equals the average distance between the centers of Earth and the Sun, or . Pluto is about 38 AU from the Sun and Jupiter is about 5.2 AU from the Sun. One light-year
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
is 63,240 AU.
See also
* List of plasma (physics) articles
References
External links
{{wikinews, NASA mission to map the boundary of solar system
* List o
Neutral Atom References
* List o
Planetary science
Plasma physics
Geophysics
Image sensors
Space plasmas